18 DEFINITION OF THE TERM INSECT. 



is subject when young ; legs most commonly reduced to 

 six. Arachnida: Distinguished from Crustacea by 

 having their respiratory organs always internal, opening 

 07i the sides of the abdomen or thorax to receive the re- 

 spirablefuid. Sometimes these organs perform the office 

 of lungs, and then the circulation takes place by means of 

 a dorsal vessel, which sends forth arterial, and receives 

 venose branches. Sometimes they are trachea or air- 

 vessels, which, as in the class Insecta, replace those of 

 circulation. These have only the vestige of a heart, or a 

 dorsal vessel alternately contracting and sending forth 

 no branch. The absence of antennae, the reunion of the 

 head with the thorax, a simple trachea but ramified and 

 almost radiating, serve to distinguish these last Arachnida, 

 or the most imperfect of insects, which respire only by 

 trachea*." Under this head he observes — " Of all 

 these characters, the most easy to seize and the most 

 certain would doubtless be, if there were no mistake in it, 

 that of the absence of antennae ; but later and compara- 

 tive researches, confirmed by analogy, have convinced 

 me, that these organs, under particular modifications it 

 is true, and which have misled the attention of naturalists, 

 do exist b :" and he supposes, from the situation and di- 

 rection of the mandibles of the Arachnida, corresponding 

 with that of the intermediate pair of antennce in Crustacea, 

 that they really represent the latter organs. If this sup- 

 position be admitted, their use is wholly changed; the palpi, 

 in fact, executing the functions of antennas, which proba- 

 bly induced Treviranus to call them Filhlhorner (Feeling- 



a Des Rapports gen'eraux, §c. det Anim. invertebr. artic, Ann. du 

 Mus. 



b Ibid. Hor. Entomolocr. 383, 



